The correct answer is C, but i don't know why. I think it can be "BeginRunEnd", but also "RunBeginEnd" in case thread "a" succeds to print "Run" before the "main" thread prints "Begin". Do you have idea why the only correct answer (according to the site) is C?

On a multiple-choice exam, one must find one "correct" answer and four "incorrect" answers. "C" is correct, but the other four are incorrect. It is a bit like the following question about swimming-pools:

What might happen if you dive into a swimming-pool full of cold water?

A: You remain dry

B: You suffer flame burns

C: Your swimming costume comes off

D: You hit your head on an ocean liner

E: You are eaten by an Ichthyosaurus

The answer is obviously C, because you cannot stay dry or suffer flames burns in a swimming-pool, and ocean liners or Ichthyosauri are never found in swimming-pools. There are all sorts of other possible right answers, eg

You get wet

All the girls think you are manly and attractive

All the men think you are feminine and attractive

You win a prize for diving

You swim away

. . . but those are not given as possibilities. So "your swimming custome comes off" is the only possible correct answer available for that question.

Interesting analogies coming from Campbell these days Or is it the usual?

But agree- with MCQs we need not worry about the answers which are not given as options. But always better to read the question carefully as you may end up selecting the right answer, but you were asked to select the wrong answers